1. Field of the Invention
The invention relates to transmissions. In particular, the invention relates to split-axis transmissions for vehicles.
2. Related Art
Transmissions provide varying torque ratios between engines or motors and the drive axles to provide efficiency and higher performance in vehicles. Manual transmissions require the operator to recognize when a change in torque ratio would be beneficial, and then to shift elements of the transmission to effect a ratio change. Manual operation of transmissions may be cumbersome to some operators. Further, power is lost during the time that the engine or motor is disengaged while the transmission elements are shifted to effect the next desire ratio. To address these issues, several improvements have been made on the manual transmissions traditionally used in vehicles. For example, the automatic mechanical transmission was developed to automatically determine when a shift was necessary, and then to shift the transmission elements to achieve a desired torque ratio. However, the automatic mechanical transmission, like the manual transmission, suffers from torque interruption during shifting: the transmission must first shift into neutral (i.e., disengage the engine drive torque from the transmission) before changing the gear ratio.
The powershift automatic transmission is an improvement over automatic mechanical transmissions in that it does not require a decoupling of torque during the ratio change. The powershift transmission provides an overlap of ratios during the ratio change so that both the shift time and the disturbance in power is minimized. Nonetheless, powershift automatic transmissions still suffer from power losses associated with released multi-plate clutches and pumping losses for lubrication and clutch actuation flow.
The dual-clutch transmission reduces the overall ratio changing time, since an overlap can be performed during the shift, which results in less disturbance of torque during a shift. Dual clutches installed between the transmission input and output provide a means to operate parallel torque shafts. A controller preselects the next gear using, for example, a predictive algorithm that considers the current gear ratio, acceleration, speed, or other variables. When the transmission shifts to the next gear ratio, one clutch disengages the transmission from the current gear while the other clutch engages the transmission in the target gear. Thus, power flows almost constantly from the drive mechanism to the wheels of the vehicle. The shift quality of the dual-clutch transmission can theoretically match that of a powershift automatic transmission. Dual-clutch transmissions have the advantage of eliminating the multi-plate clutches used in a powershift automatic, and by using a much smaller oil pump. These two changes provide reductions in parasitic losses. Further enhancements in efficiency may be gained with a dual-dry clutch, which eliminates the need for a torque converter. However, the shift quality and driveability may be reduced as a result. Another disadvantage of eliminating the torque converter is the loss of torque multiplication associated with the device.
Major disadvantages of dual-clutch transmissions are cost, launch consistency, and complexity of control strategy. Dual-clutch transmissions are able to preselect only one gear ratio at a time. If the transmission incorrectly predicts the next gear ratio, then excessive delays in the shift event may occur, and the transmission may be forced to remain in neutral for an extended period of time. This results in power loss and a degraded driving experience.
Thus, there is a need for a highly efficient, cost-effective transmission with improved capabilities for preselecting a target gear and for minimizing torque interruption during a gear change.